When selecting equipment for bulk material handling, one of the most common mistakes is focusing solely on the volumetric capacity of the equipment. However, the factor that most conditions that choice, and is most frequently overlooked, is the density of the material to be moved.
True density and bulk density: they are not the same
Before discussing selection criteria, it is worth clarifying a concept that causes a great deal of confusion in the industry: the difference between true density and bulk density.
True density refers to the mass per unit volume of the material itself, without accounting for the void spaces between particles. It is a laboratory value, useful for characterising the material, but of little practical use when sizing loading equipment.
Bulk density (also known as apparent density or volumetric weight) measures the mass per unit volume of the material as it exists in reality: with its voids, its granulometry, and its level of compaction. This is the value that truly determines how much what fits in a grab or a bucket actually weighs.
The difference between the two can be enormous. Dry sand has a true density of approximately 2,600 kg/m³, but its bulk density is around 1,500 to 1,600 kg/m³ depending on how it has been deposited. Bulk cement can range from 1,000 to 1,500 kg/m³ depending on its condition. Ignoring this difference can lead to overloading the equipment or, conversely, to operating far below its useful capacity.
Why density determines the equipment
A loading equipment has two operating limits that must be respected simultaneously: the volumetric limit (how many cubic metres fit in the grab or bucket) and the load limit (how many tonnes the carrying equipment, whether a crane, excavator, or overhead crane, can safely lift).
When the material is low-density, such as biomass, sawdust, or shredded plastic, the equipment reaches the volumetric limit before the load limit. In this case, it makes sense to maximise the grab's volume, and lower load-capacity equipment can be used.
When the material is dense, such as scrap metal, coarse aggregates, minerals, or pig iron, the load limit is reached before the equipment is full. Here, using a large-volume grab is counterproductive, as the operator will be forced to work with it only partially filled in order not to exceed the maximum load, resulting in a drop in performance and the risk of compromising the equipment's structural integrity.
The impact on solid bulk transport
This logic has direct implications for any bulk solid transport operation, whether at port, at a plant, or on site. Equipment that is incorrectly sized relative to the material's density generates slower cycles, higher energy consumption, and in many cases, accelerated wear of the mechanical and hydraulic components.
For example, a clamshell grab designed for medium-density materials (between 1,000 and 1,500 kg/m³) that is regularly used with high-density materials such as gravel or granite (2,500 to 2,700 kg/m³) is systematically operating under overload conditions. Even if the equipment does not fail immediately, the cumulative damage to the cylinders, joints, and cables is significant.
Practical criteria for selection
When choosing or specifying loading equipment, it is advisable to follow these steps:
- Obtain the actual bulk density of the material, not the theoretical value. If there is variability due to moisture content, granulometry, or compaction, work with the highest foreseeable value.
- Calculate the maximum load in tonnes that the carrying equipment can handle under real working conditions (slewing radius, cycle speed, reduction factor for duty cycle).
- Size the grab volume by dividing that maximum load by the material's bulk density. That volume represents the real useful ceiling, regardless of the nominal volume of the equipment.
- Factor in the fill factor, which rarely reaches 100% in granular or irregular materials, and which also depends on the material's granulometry and flowability.
- Seek advice from specialists in bulk material handling equipment, such as electrohydraulic orange peel grabs and clamshell grabs, who can analyse the specific scenario and define real-world solutions.
How Stemm helps you choose the right equipment
At Stemm, we have spent decades designing and manufacturing clamshell grabs, orange peel grabs, and clamps for all types of materials and densities. We know that every application is different: handling wet biomass is nothing like handling heavy scrap metal, and working at a port is not the same as working at a waste treatment plant.
That is why we have developed an online configurator where you can set up your equipment based on the material, density, tonnage per cycle, and type of crane or excavator. In just a few minutes, you get a technical and commercial reference tailored to your actual operation, with no need to wait for a sales visit.
